Probing the thickness and roughness of the functional layer in thin film composite membranes

Abstract

The thickness and roughness of the functional layer in thin film composite membranes (TFCMs) are important determinants of the membrane performance, whether in reverse osmosis, gas separation or energy applications. This paper describes and compares different techniques for the measurement of these quantities. A new method based on atomic force microscopy (AFM) is developed and compared with techniques based on scanning electron microscopy (FEG-SEM) and profilometry. AFM and profilometry methods allowed a simultaneous determination of thickness and roughness and could be extended for studying the effect of parameters such as support membrane roughness. The methods have been tested on polyamide TFC RO membranes. The membranes were prepared by an interfacial polycondensation technique on polysulfone support, by reaction between m-phenylene diamine and trimesoyl chloride in the organic phase. Based on our earlier work which showed the important role of shear on membrane morphology in such syntheses, preparations both with and without shear were employed. The measurement of thickness using AFM was carried out on specially made membranes in which, by the use of a mask, only a part of the PSF was coated with the PA layer. By scanning a region that contains both the covered and the uncovered parts, thickness values could be obtained. In the second technique, the SEM cross sectional images of TFCMs allowed the determination of the distribution of thickness over the area seleted. In the third, a surface profilometer was used to determine the thickness of de-laminated PA films. The thickness distributions and average thicknesses measured from all three methods matched closely. Overall, these techniques are of great potential in TFC and similar membrane characterization.